open access

https://hdl.handle.net/2115/76125

Viscoelastic Modeling of Nocturnal Thermal Fracturing in a Himalayan Debris-Covered Glacier

英語

viscoelastic model

2019-06

American Geophysical Union

Journal of geophysical research earth surface

124(6)

1485-1515

Recent observations suggest that the nocturnal thermal fracturing of ice occurs at relatively warm temperatures (above -15 degrees C) at a high-altitude Himalayan glacier system unless the ice is shielded by a debris mantle. Here we estimate the stresses induced by diurnal temperature variations using viscous, elastic, and two viscoelastic models, and various thicknesses of the debris mantle. Only the elastic and visco-elastic models are in agreement with the observations. The timing and amplitudes of the stresses in the upper 15 cm of the glacier are different among the models despite the ability of each approach to predict a diurnal increase in tension exceeding the critical threshold proposed for crevasse formation. For example, the elastic stress is several times larger than the viscous stress at the ice surface (650 vs. 250 kPa) and reaches its peak up to 5-6hr later in the night. The time lag is in line with the seismic records, suggesting that the viscous model is not appropriate. Furthermore, a debris layer of >= 50cm in thickness suppresses the diurnal fluctuations in thermal stress and therefore protects the ice from mechanical damage. We suggest that high-amplitude diurnal cooling and weak ice properties due to weathering are essential factors that influence thermal fracturing in the Himalayan environment. The ongoing expansion of seismic networks into cryospheric regions, which will be capable of detecting local thermal-contraction-induced cracks, in combination with the fact that such cracks can erode and weaken the ice, and thereby serve as meltwater and heat channels, warrants further research to better understand these near-surface processes and to monitor ice properties. Plain Language Summary Thermal fracturing is an important erosion process on the icy surfaces of solar system bodies, including Earth, Mars, and comets. However, the exact timing of this thermal fracturing process is poorly constrained, despite the need for this information to validate models, and its importance in the weathering of glacial ice is largely unknown and often overlooked. Recent seismic observations revealed nocturnal thermal fracturing of a high-altitude Himalayan debris-covered glacier. These observations in Nepal suggested that glacial ice bursted with icequakes as temperature decreased unless ice was protected with a thick debris cover. In this study we consider four different numerical approaches to describe material behaviour of ice under thermal stress in order to find out how debris modulates stresses and which method agrees with the experimental evidence. We numerically estimate thermal stress conditions near the surface of a glacier with and without debris and find that a half-of-a-meter-thick debris is sufficient to protect ice from mechanical damage induced by diurnal variation of temperature. Furthermore, our study suggests that thermal stress could be an important factor for weathering of exposed ice and that this process needs more attention since it is possible that new cracks can facilitate ablation.

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